The availability of large quantities of natural gas has led to numerous proposals to upgrade the gas to more valuable chemicals including oxygen containing orgarnic compounds. The only commercial processes so far developed have required the sequential steps of catalytic dehydrogenation of the alkane to form an olefine followed by oxidation of the reactive olefinic site to produce the oxygenate. Other processes have been proposed for the direct oxidation of alkanes to oxygenates however these have low conversion and low selectivity and usually require high temperatures.
The two stage vapour phase oxidation of propylene for the production of acrylic acid is known to the art. However, there is no commercial process that exists based on propane oxidation to acrylic acid. The production of acrylic acid from propane would be more attractive because of the significant price difference between propane and propylene and the elimination of conversion steps.
There are few references reported in the literature relating to the production of acrylic acid from propane. U.S. Pat. No. 5,198,580 discloses a process for partial oxidation of propane to yield acrylic acid, propylene, acrolein, acetic acid and carbon oxides by the reaction of propane in admixture with a molecular oxygen-containing gas in a reaction zone with a catalyst containing Biz, Moc, Vv, Aa, Dd, Ec, Ox; where A is one or more of K, Na, Li, Cs and Tl; D is one or more of Fe, Ni, Co, ZXn, Ce and La; E is one or more of W, Nb, Sb, Sn, P, Cu, Pb, B, Mg, Ca and Sr, values for a, d and e are from 0 to 10, b is from 0.1 to 10, c is from 0.1 to 20, v is from 0.1 to 10, c:b is from 2:1 to 30:1 and v:b is from 1:5 to 8. The acrylic acid yield achieved using the bismuth molybdate type of catalyst is 4.5% at 19% conversion of propane at a pressure of 20 psig and a temperature of 400° C.
European Patent EP 0608838 A2 to Takashi et al discloses a method of producing an unsaturated carboxylic acid, mostly in the explosive regime of the propane, air and water mixture at 380° C. in the presence of a catalyst containing a mixed metal oxide of MoVTeXO, wherein X is at least one element selected from bismuth, cerium, indium, tantalum, tungsten, titanium, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum and antimony, wherein the proportion of the respective essential components are based on the total amount of the essential components exclusive of oxygen and satisfy the following formulae: 0.25<Vmo<0.98, 0.003<Vv<0.5, 0.003<Vx<0.5, wherein Vmo, V, VTe and Vx are molar fractions of Mo, V, Te and X. Recently, Takashi et al disclosed in another JP Patent Number 1045643 (9845643-February 1998), the formation of acrylic acid and acrolein in the presence of PaMobVcWdXeOn (X—Nb,Ta, Ti, Zr, Sb; if a=1 then b=1-18, c=0-4, d=0-4 and e=0.05-20) at 380° C. achieving a yield 0.9% to acrolein and 3.5% to acrylic acid at 12% propane conversion
U.S. Pat. No. 6,646,158 suggests the use of a catalyst with a calcined composition of MOa, Vb, Gac, Pdd, Nbe, Xf, wherein X=at least one element selected from the group consisting of La, Te, Ge, Zn, Si, In and W; a is 1; b is 0.01 to 0.9; c is >0 to 0.2; d is 0.000000001 to 0.2; e is >0 to 0.2; and f is >0 to 0.5 for the oxidation of propane to acrylic acid and acrolein. The numerical values of a, b, c, d, e and f represent the relative gram-atom ratios of the elements Mo, V, Ga, Pd, Nb and X respectively in the catalyst. The elements are preferably present in combination with oxygen in the form of various oxides.
The above referenced catalysts disclosed in the literature result in low yields of acrylic acid at relatively high temperatures and most produce propylene as one of the significant by-products. Propylene can be expensive and difficult to separate, especially in a recycling mode of operation.
Further examples of the mixed metal oxide component of the catalyst for the production of acrylic acid in one step by subjecting propane to a vapour phase catalytic oxidation reaction are a Mo—S-b-P—O type catalyst (European Patent No. 0010902), a CV—P—Te—O type catalyst (Journal of Catalysis, Col 101, p389 (1986)), a Bi—Mo—O type catalyst and a V—P—Te—O type catalyst (Japanese Unexamined Patent Publication No. 170445/1991). On the other hand, as an example of the catalyst for the production of methacrylic acid in one step by subjecting isobutene to a vapour phase catalytic oxidation reaction, a P—Mo—O type catalyst (Japanese Unexamined Patent Publication No. 145249/1988) is known.
However, each of the methods using such catalysts has a drawback such that the yield of the desired unsaturated carboxylic acid is not adequate or the reaction system is complex.
We have now developed a process that enables alkanes to be oxidised under less severe conditions and with a greater selectivity.